Modern vehicles are typically controlled and monitored by multiple Electronic Control Units (ECUs) that coordinate their operations by communicating over one or more internal network buses. In addition, modern vehicles are becoming ever more connected through external network interfaces, such as those supporting Radio-frequency Identification (RFID), Bluetooth, Dedicated Short Range Communications (DSRC), Wi-Fi, and cellular communications protocols. This connectivity, on the one hand, facilitates a variety of services including telematics, navigation and safety that provide significant benefits for automakers, aftermarket vendors, fleet managers and passengers. But on the other hand, these capabilities introduce new security and privacy concerns. For example, researchers have highlighted the vulnerability of modern vehicles to cyber-attacks, such as by evading vehicle network defenses and infecting ECUs with malware to control a wide range of essential vehicle functions. Various intravehicular mechanisms for improving security have been proposed by the industry and academia. However, such intravehicular mechanisms are typically limited by inadequate computational and memory resources, access to data within the vehicle and in the vehicle's immediate vicinity only, and often sporadic, unreliable and expensive connectivity with external service providers.